The present invention relates to an apparatus for treating a sample by a pulsed electron beam and is used more particularly in the treatment of the surface layers of semiconductor materials.
Thus, on implanting foreign particles or impurities e.g. in a semiconductor substrate by carrying out a bombardment of the latter, it is known that following the said implantation it is necessary to perform a stage called annealing enabling the implanted impurities to be made electrically active and enabling the rearrangement of the crystal lattice of the substrate, which was subject to interference during the bombardment.
One of the most frequently used methods for carrying out this annealing stage is to raise the implanted substrate to a high temperature (approximately 900.degree. to 1200.degree. C.) for a certain time.
Another more recent method consists of briefly applying a high energy density to the surface or at the level of the first implanted layers in such a way that very high temperatures are locally reached during this time. In certain cases the temperatures reached make it possible to liquefy the first layers of the substrate, thereby curing the damage produced during implantation. Due to the fact that the temperature rise is very localized and of short duration (below 1 millisecond) the remainder of the substrate is not affected.
The energy can be supplied to the surface or the first layers of the substrate either by means of a light energy burst using e.g. a laser or a flash tube, or by means of an intense beam of particles such as electrons. These particle or light beams can be pulsed or unpulsed narrow or wide beams and can sweep over the substrate in such a way as to "anneal" the latter at different points.
Known apparatus for treating samples by means of intense, pulsed electron beams generally comprise a field-plasma emission diode of the type shown in FIG. 1.
This apparatus comprises a diode constituted by a generally graphite cathode 2, provided with a plurality of grooves, and an anode 4 constituted by an actual anode 4a and an grid 4b, the two elements 4a and 4b being raised to the same potential. This diode is generally placed in a vacuum enclosure 6.
Moreover, such apparatus generally comprise a high voltage generator 8 connected to the high voltage energy storage system 10 which can be formed, for example, by a coaxial line or a capacitor, as well as a pulse triggering switch 14, such as a spark gap switch. Such apparatus also comprise a supply system 16 supplying the samples 18 to be treated to the vacuum enclosure 6, and systems 20 enabling the measurement of the voltage and current supplied by the generator 8 over a period of time. The actual anode 4 a serves as a support for the sample 18.
Such apparatus have been described, for example, in the Kirkpatrick U.S. Pat. No. 3,950,187 entitled "Method and apparatus involving pulsed-electron-beam processing of semiconductor devices", as well as in an article in the Journal of Applied Physics, vol. 50, no. 2, February 1979 entitled "Pulsed-electron-beam annealing of ion implantation damage".
In such apparatus, the application of a high voltage between the anode grid 4b and cathode 2 by means, for example, of a capacitor 10, previously charged by the generator 8, makes it possible to produce an intense electric field in the vicinity of the diode cathode 2. The application of the charging voltage of capacitor 10 is ensured by means of the spark gap switch 14 controlled by the pulse triggering system 12.
The electric field produced is intensified in the vicinity of the cathode by the presence of whiskers, resulting from the not shown grooves on the cathode. These whiskers lead to the emission of an electric field. The electric power supplied in this way is such that there is an explosion of these whiskers corresponding to an explosive vapourization and an ionization thereof. The plasma microspheres produced in this way in turn become an electron source favouring the rapid rise in the current, which itself aids the explosion. In a few nanoseconds, each of the plasma microspheres extends sufficiently for the cathode to be covered with a continuous plasma envelope or shell. The thickness of the latter increases due to expansion until it reaches the anode grid, producing a diode short-circuit, known as a diode-closure.
The capacitor 10, charged by generator 8, continues to discharge into the diode. The voltage applied between the anode and the cathode by the capacitor has made it possible, prior to the short-circuit taking place, to extract and accelerate the electrons produced so as to perform an intense, pulsed electron beam which can be used e.g. for the annealing of sample 18.
The fact that such an apparatus uses the actual anode 4a serving as a support for sample 18 leads to the positive polarization of the sample and enables the electrons to easily penetrate the latter, whereas the ions formed cannot penetrate the sample and are automatically repelled. Thus, an intense, pulsed electron beam is indeed obtained.
Therefore, via capacitor 10, the high voltage generator 8 is used both for producing the plasma shell acting as the electron source, as well as for extracting and accelerating the latter.
Such apparatus make it possible to produce electron beams, whose energy is between 10 and 50 kiloelectron volts (KeV), whose intensity is between 100 and a few thousand A/cm.sup.2 and whose pulse length is between a few dozen nanoseconds and a few microseconds.
Further details on the operating principle of such apparatus can be obtained from the article in the Journal of Applied Physics, vol. 45, no. 6, June 1974 entitled "Plasma-induced field emission and the characteristics of high-current relativistic electron flow".
However, these simple apparatus have a certain number of disadvantages, which are in particular:
the need of suddenly producing a high voltage in the space between the anode and the cathode under conditions such that the intensity must be very high, i.e. between 100 and 10,000 amperes; PA1 the characteristics of the electron beam, i.e. its energy and current density, cannot be selected independently of the conditions under which the plasma is produced, thus e.g. if it is desired to obtain a low energy electron beam, i.e. of approximately 20 KeV, it does not stand to reason that the corresponding electric field will be sufficient for producing the plasma shell; PA1 the use of a very high power system for producing the plasma, although this is not required for producing the latter. PA1 (A) a first circuit for producing a plasma in pulsed manner from the cathode of the vacuum tube, said first circuit comprising: PA1 (B) a second circuit for producing between the actual anode and the cathode an electric field making it possible to accelerate and extract the electrons produced in such a way that they bombard the sample, said circuit comprising a capacitor of capacitance C.sub.2 well above capacitance C.sub.1, said capacitor of capacitance C.sub.2 being charged by means of a high d.c. voltage generator and able to discharge by the actual vacuum tube.